The present invention pertains generally to A. C. protective circuits for protecting electronic devices from excessive A. C. voltages and specifically to such a circuit which may be advantageously utilized to protect the input circuitry of a radio receiver.
As is well known, R. F. signals of excessive intensity can damage the sensitive low level R. F. input circuitry of radio receivers unless proper protection is afforded. Receivers can be subjected to these excessive R. F. signals when operating in close proximity to a non-related transmitter whose transmission is inadvertently picked up by the radio receiver antenna or by a malfunction in the antenna switching device for controlling a receiver antenna that is commonly utilized for both transmitting and receiving. Although there have been a number of solutions developed to overcome this problem, these all exhibit one or more inherent disadvantages. For example, when clamping is employed to hold the input voltage to a desired level, significant amounts of power may have to be dissipated in the clamping device and when commonly used in combination with a series current limiting resistor attenuation in the desired signal results. A second approach which has been adopted and which is exemplified in U.S. Pat. No. 3,870,899 entitled "Receiver Protection Circuit Apparatus" entails monitoring the input voltage for the purpose of deactivating a series switch to prevent the application of high voltages to the sensitive circuitry which follows it. As may be perceived from this one example of this type of approach, this solution requires copious circuitry which materially adds to the complexity and cost of the radio receiver.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved A. C. protective circuit for protecting electronic devices from excessive A. C. voltages.
It is further object of the present invention to provide such an A. C. protective circuit which is simple in design and operation, yet provides effective protection.
It is still a further object of the present invention to provide such an A. C. protective circuit which is ideally suited for use with a radio receiver to protect it from excessively intense R. F. signals which might otherwise damage equipment therein.
These objects, as well as others, and the means by which they are achieved through the present invention may best be appreciated by referring to the Detailed Description of the Invention which follows hereinafter together with the accompanying figures.